This invention relates to fuel cells (i.e., cells in which the cathode material is constantly supplied or available, such as air depolarized cells, having non-consumable cathode elements and constantly supplied air (specifically oxygen) as the oxidizing cathode material. The invention particularly relates to metal anode fuel cells having a large metal anode to cathode electrochemical ratio and with anodes in the fluid or permeable fluid/paste state.
Generally battery systems have a low gravimetric electrochemical capacity of less than about 200 Whr/kg. Metal fuel cells such as zinc/air cells are however among the highest capacity systems but are generally suited only for extremely low rate application such as remote signal light, hearing aid, and communication system applications.
Despite the high capacities, conventional metal-fuel systems have been characterized by low rate capability as a result of poor cathode performance and availability of limited active cathode depolarization sites. In addition, anode utilization is minimized relative to the total amount of metal anode since effective discharge is confined to a limited surface depth of the anode. Thus, very thick anodes will develop an increasingly larger internal resistance loss when the oxide layer on the anode increases as a result of progressive cell discharge. Accordingly there is a maximum anode thickness, for reasonable operation, of generally only a few millimeters. As discharge current or rate increases, even this small effective thickness is further reduced.
Expedients used to increase utilization capacity of metal air cells have generally embodied fixed cathodes and movable anodes (i.e., fresh anode materials such as tapes or plates are inserted into the cells as reaction products are removed) to enhance cell capacity. Alternatively, as described in co-pending application Ser. No. 09/570798, filed May 12, 2000 (the disclosure of which is incorporated herein by reference thereto), anode material is in the form of a dispensable or flowable paste which is continuously introduced into the cells as reaction products are exhausted therefrom. While effective, this latter system generally requires metal paste transport means such as a pump, as well as a storage system for continuous operation.
It is an object of the present invention to enhance cell capacity and performance of fuel cells, and more particularly fuel cells with metal containing fluid or paste-like anodes and most particularly metal-air cells while eliminating the need for anode movement and movement-inducing elements by using a fixed anode paste pool and a cathode which is movable within the anode material.
It is a further object of the present invention to enhance volumetric efficiency of the cells by eliminating a pump as used for movement of anode paste.
It is yet another object of the present invention to increase efficiency in the utilization of an anode paste with a movable cathode, with decreased expended energy, while increasing energy density and cell discharge capacity.
It is still yet another object of the present invention to provide a cell with the movable cathode being propelled as a function of the electrochemical cell discharge and reaction.
Generally the present invention comprises a fuel cell with a non-consumable cathode, such as an air depolarized cell, having at least one cathode element disposed in an anode material through which it can be moved. Reaction products (in air depolarized cells the reaction product is an oxide of the anode metal) are often removed from the cell as an exhaust, as new anode material is supplied to the cell usually from a reservoir (the supply or air or oxygen, of course, remains constant). The anode materials are generally comprised of flowable particles of a metal such as of alkali and alkaline earth metals or transition metals such as nickel, iron and zinc and varying alloys thereof. Zinc is most preferred for aqueous cells, and lithium for non-aqueous cells. The anode is preferably in the form of a pool of a fluid or paste material exemplified by a slurry or paste of zinc and electrolyte.
The electrochemical capacity of the anode is the limiting factor in the cell capacity and is adapted to exceed the fixed capacity of the cathode, preferably by several factors (either in the cell itself or as a constant supply from an external reservoir. The cathode element(s), with appropriate enwrapping ionically permeable separator material (for maintaining structural integrity of the cathode and for preventing a short circuiting between anode and cathode), is adapted to effectively move, be caused to move, or xe2x80x9cswimsxe2x80x9d within the anode material, to maintain an electrochemical proximity of active anode material and the active cathode depolarizing element.
In air depolarized cells the cathode element generally comprises a non-consumable element comprised of an electrically conductive material such as carbon affixed to a conductive grid. In a preferred embodiment, the cathode element is contained within a streamlined container element, such as of an ovoid horizontal cross section shape to reduce drag and to facilitate cathode movement within the anode. For maximum utilization efficiency the height of the cathode is substantially matched to the depth of the anode paste. The cathode and the placement thereof within the anode is adapted to permit air access to the cathode from an open and exposed upper end for effective depolarization, during cathode movement. The cathode moves at a rate sufficient to effect substantially complete and effective utilization of proximate anode material and to constantly maintain electrochemical coupling with fresh anode material.
For maximum volumetric efficiency, each cell is preferably of flat shape and is comprised of an air-diffusion cathode, a separator, and a nickel-based current collector, and the metal paste. Examples of separator materials are disclosed in co-pending application U.S. Ser. No. 09/259,068 filed Feb. 26, 1999, relating to conductive polymer gel membrane separators wherein anion- and cation-conducting membranes are formed. The cathode is effectively covered or wrapped in the separator material. The gel composition of the membrane contains the ionic species within its solution phase such that the species behaves as in a liquid electrolyte, while at the same time, the solid gel composition prevents the solution phase from diffusing into the device. Other useful separator materials are disclosed in co-pending application U.S. Ser. No. 09/482,126, filed Jan. 11, 2000, wherein a separator is disclosed which comprises a support or substrate and a polymeric gel composition having an ionic species contained in a solution phase thereof. In preparing the separator, the ionic species is added to a monomer solution prior to polymerization and remains embedded in the resulting polymer gel after polymerization. The ionic species behaves like a liquid electrolyte, while at the same time, the polymer-based solid gel membrane provides a smooth impenetrable surface that allows the exchange of ions for both discharging and charging of the cell. Advantageously, the separator reduces dendrite penetration and prevents the diffusion of reaction products such as metal oxide to remaining parts of the cell. Furthermore, the measured ionic conductivity of the separator is much higher than those of prior art solid electrolytes or electroyte-polymer films.
A suitable cathode structure is described in co-pending application U.S. Ser. No. 09/415,449, filed Oct. 8, 1999, comprised of a porous metal foam substrate, formed with a network of interconnected pores. An active layer and a hydrophobic microporous gas diffusion layer are both disposed on one or more surfaces of the metal foam substrate. The metal foam substrate serves as the current collector of the cathode. The microporous layer is a plastic material such as a fluoropolymer (i.e., PTFE). The cathode may also include a particulate microstructure reinforced by relatively strong bonding provided by sintering a polymeric binder within the three-dimensional interconnected porosity of the metal foam substrate. The reactive layers are preferably fabricated from the same material as binder. This advantageously enables a single roll pressing operation to simultaneously impregnate the binder into the substrate and form the reactive layers thereon. The disclosures of said applications are incorporated herein by reference thereto.
In a preferred embodiment, cathode movement means, such as a wire or rod control, mechanically laterally moves the cathode through the anode material. Activation of the cathode movement means is coupled to a voltage drop sensor in the anode-cathode couple. When a sharp drop, indicative of essentially complete utilization of adjacent anode material, particularly in a zinc air cell, is detected, the cathode movement means are activated for a time sufficient to move the cathode to an adjacent fresh proximity with anode material. In a preferred embodiment of a zinc air cell, the characteristics of the cell provide both the detection and the propulsion means for the cathode wherein a change in volume of the cell reaction materials serve to propel the cathode forward as it is formed. The cells utilizing the present invention range from small cells used to power small appliances in the range of several watts of contained power, to large electrical supply cells of megawatt power levels.
The above objects, features and advantages of the present invention will become more evident from the following discussion and the drawings in which: